Surface flame spread

This Issue This is a part of the Exteriors and interiors feature

By - , Build 166

Research is under way at BRANZ to improve knowledge and management of surface flame spread. This fundamentally important aspect of fire science is often critical to the ultimate outcome of a fire in a building.

SURFACE FLAME SPREAD is the process where flames spread across the continuous surface of a fuel. It is not to be confused with fire spread, the process where a fire spreads from one discrete fuel item to another.

Fire science behind surface ignition

When the surface of a fuel is subjected to external heating, the material’s chemical structure will change and start to undergo a phase change from a solid or liquid to a gas. In fire science, this process is described as off-gassing or pyrolysis.

The pyrolysis gases mix with oxygen above the surface of the fuel, and once that fuel/ oxygen mixture reaches a certain critical level called the flammability limit, ignition can occur. If there is a suitable ignition source present, such as an electrical spark, the mixture will ignite.

Alternatively, if subject to further external heating the temperature of the mixture can reach the threshold at which it will spontaneously ignite.

External heat and fuel needed

If there is enough energy present to produce a flammable mixture, the fire will grow and the flame will spread across the surface of the fuel. Surface flame spread is in effect a continuous piloted ignition. The flame is the pilot ignition source for the flammable gases and part of the external heat source for pyrolysis of the fuel in front of the flame spreading outwards from the ignition point.

The rate of pyrolysis depends on the amount of external heat that the surface of the fuel receives and the nature of the fuel. For example, a volatile liquid such as petrol produces gases at its surface at ambient temperatures without any external heating and a flaming match above the surface ignites the fuel/air mixture very easily.

In contrast, holding a flaming match at the surface of a large piece of dry timber is unlikely to cause ignition. This is because the timber requires significantly more external heating to produce a flammable fuel/gas mixture than can be provided from a flaming match.

Thermal inertia, which is the ability of the fuel to absorb and store energy, also has an impact on its ignition. If the material has a high thermal inertia, it will be more difficult to ignite, and vice versa for the same amount of external heating.

The reason is that a material with a high thermal inertia will act as a heat sink, absorbing energy that would otherwise be converting the fuel into pyrolysis gases. The fuel thickness also affects ignition. For example, compare a piece of timber to shavings from the same piece that can easily be ignited by a flaming match.

Although any thickness of fuel has the same thermal inertia, as these are material properties independent of thickness, the capacity of the shavings to act as a heat sink is reduced, and it will ignite more easily. Similarly, the same piece of timber will take longer to ignite if it is wet. Prior to the formation of flammable gases, it takes energy to drive off the moisture, thus delaying ignition.

Rate of surface flame spread

The rate of surface flame spread is one aspect that will affect the ultimate outcome of the fire event. If the rate of surface flame spread is relatively low, the chances of building occupants safely self-evacuating in the event of a fire are likely to be higher, and vice versa.

This is because the rate of surface flame spread will govern how quickly flames, smoke and gases are produced and how quickly a fire reaches an out-of-control stage where building occupants are less likely to escape. Exactly the same factors that affect ignition – thermal inertia, thickness and moisture content – will have an impact on the rate of surface flame spread.

Interior vs exterior flame spread

Surface flame spread is governed by how much surface area of fuel is present and, within a building, how much oxygen is available. Interior surface flame spread is generally only significant in the earlier stages of a fire and limited to the area of the building where the fire is initiated.

Within a building, the heat from a growing fire does not dissipate as freely as in the open, and this trapped heat enhances the pyrolysis process and the surface flame spread. Heat retention is also affected by the insulating properties of the building – the better the insulation, the less heat is lost and the more enhancement can occur.

Once the trapped heat reaches a critical level, a fire within a building often goes through a rapid transition called flashover where all the fuel in a space in a building is involved in the fire. At this point, surface flame spread has ceased to govern the fire growth. Eventually, the fire will outgrow the amount of oxygen that can enter through openings in the compartment or spread to the maximum extent of fuel surfaces available.

Different when outside

The issue of surface flame spread is different in exterior situations. Instead of affecting the development of the fire within a specific area of a building, external surface flame spread is a question of the fire spreading from one floor of a building to another.

A fundamental principle of building fire safety is that the fire is contained within the area where it first breaks out. For a multi-storey building, this means that the fire should not spread from one floor to another.

If the external cladding on a building is combustible, this can provide a pathway for the fire to spread vertically to upper floors of the building via surface flame spread on the cladding.

Controls on fire spread

There are regulatory controls for both internal and external fire spread. These limit the combustibility of internal wall and ceiling linings and floor coverings to limit the growth of internal fires during the pre-flashover stage in high risk and critical areas of buildings. They also limit the combustibility of external surfaces for certain building configurations.

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